A liquid transfer article, such as a roll, is used in the printing industry to transfer a specified amount of a liquid, such as ink or other substances, from the liquid transfer article to another surface. The liquid transfer article generally comprises a surface with a pattern of depressions or wells adapted for receiving a liquid and in which said pattern is transferred to another surface when contacted by the liquid transfer article. When the liquid is ink and the ink is applied to the article, the wells are filled with the ink while the remaining surface of the article is wiped off. Since the ink is contained only in the pattern defined by the wells, it is this pattern that is transferred to another surface.
In commercial practice, a wiper or doctor blade is used to remove any excess liquid from the surface of the liquid transfer article. If the surface of the coated article is too coarse, excessive liquid, such as ink, will not be removed from the land area surface of the coarse article thereby resulting in the transfer of too much ink onto the receiving surface and/or on the wrong place. Therefore, the surface of the liquid transfer article should be finished and the wells or depressions clearly defined so that they can accept the liquid.
A gravure-type roll is commonly used as a liquid transfer roll. A gravure-type roll is also referred to as an applicator or pattern roll. A gravure roll is produced by cutting or engraving various sizes of wells into portions of the roll surface. These wells are filled with liquid and then the liquid is transferred to the receiving surface. The diameter and depth of the wells may be varied to control the volume of liquid transfer. It is the location of the wells that provides a pattern of the liquid to be transferred to the receiving surface while the land area defining the wells does not contain any liquid and therefore cannot transfer any liquid. The land area is at a common surface level, such that when liquid is applied to the surface and the liquid fills or floods the wells, excess liquid can be removed from the land area by wiping across the roll surface with a doctor blade.
The depth and size of each well determines the amount of liquid which is transferred to the receiving surface. By controlling the depth and size of the wells, and the location of the wells (pattern) on the surface, a precise control of the volume of liquid to be transferred and the location of the liquid to be transferred to a receiving surface can be achieved. In addition, the liquid may be transferred to a receiving surface in a predetermined pattern to a high degree of precision having different print densities by having various depth and/or size of wells.
Typically, a gravure roll is a metal with an outer layer of copper. Generally, the engraving techniques employed to engrave the copper are mechanical processes, e.q., using a diamond stylus to dig the well pattern, or photochemical processes that chemically etch the well pattern.
After completion of the engraving, the copper surface is usually plated with chrome. This last step is required to improve the wear life of the engraved copper surface of the roll. Without the chrome plating, the roll wears quickly, and is more easily corroded by the inks used in the printing. For this reason, without the chrome plating, the copper roll has an unacceptably low life.
However, even with chrome plating, the life of the roll is often unacceptably short. This is due to the abrasive nature of the fluids and the scrapping action caused by the doctor blade. In many applications, the rapid wear of the roll is compensated by providing an oversized roll with wells having oversized depths. However, this roll has the disadvantage of higher liquid transfer when the roll is new. In addition, as the roll wears, the volume of liquid transferred to a receiving surface rapidly decreases thereby causing quality control problems. The rapid wear of the chrome-plated copper roll also results in considerable downtime and maintenance costs.
Ceramic coatings have been used for many years for anilox rolls to give extremely long life. Anilox rolls are liquid transfer rolls which transfer a uniform liquid volume over the entire working surface of the rolls. Engraving of ceramic coated rolls cannot be done with conventional engraving methods used for engraving copper rolls; so these rolls must be engraved with a high energy beam, such as a laser or an electron beam. Laser engraving results in the formation of wells with a new recast surface about each well and above the original surface of the roll, such recast surface having an appearance of a miniature volcano crater about each well. This is caused by solidification of the molten material thrown from the surface when struck by the high energy beam.
The recast surfaces may not significantly effect the function of an anilox roll because the complete anilox roll is engraved and has no pattern. However, in gravure printing processes where a liquid transfer pattern is required, the recast surfaces cause significant problems. The major difference between a gravure roll and an anilox roll is that the entire anilox roll surface is engraved whereas with a gravure roll only portions of the roll are engraved to form a predetermined pattern. In order for the gravure roll to transfer liquid in a controlled manner determined by the pattern, fluid has to be completely wiped from the unengraved land area by a doctor blade. Any fluid remaining on the land area after running under the doctor blade will be deposited on the receiving product where it is not wanted. With a laser engraved ceramic roll, the doctor blade cannot completely remove liquid from the land area due to the recast surfaces which retain some of the liquid. Thus the recast surfaces should be removed for most printing applications.
When using laser techniques to produce liquid transfer articles for applications requiring printed patterns, it is extremely difficult to control the depth and size of all the wells. Specifically, the laser is generally required to be activated only where wells are required and inactivated when no wells are required. Unfortunately, the laser start and stop response is not the same response that is achieved once the laser is operating for a set period. For example, when the laser is started, the first few pulses of radiation are less than the energy content of the laser beam for pulses produced after the laser has been operating for a suitable time. This in turn results in the shape and depth of the first few wells in the surface of the article being different from consecutive successive wells formed in the surface of the article. Consequently, the wells defining the boundary of the pattern are not the same depth and/or size as the wells contained within the center of the pattern and therefore would be incapable of containing a desired volume of liquid. This results in the boundary of the pattern transferred to a receiving surface being off shaded with respect to the overall pattern. In other words, the edges of the printed pattern are somewhat fuzzy. This can result in different shades of the printed pattern being transferred to the receiving surface. Although laser techniques provide an effective means for producing wells in the surface of liquid transfer articles, the non-uniformity of the few start and stop pulses of the laser can produce an inferior quality liquid transfer article. With regard to the location of the wells, a sharp boundary line of patterns generally requires a combination of full and fractional size surface area wells to ensure that a good boundry edge definition is obtained. Without a mask, a sharp boundry edge definition cannot be achieved.
An object of the present invention is to provide a method for producing a liquid transfer article having uniform size and depth wells on its surface.
Another object of the present invention is to provide a method for producing a quality liquid transfer roll that can be used in gravure printing processes to provide printed patterns of desired shapes and shades that cannot effectively be obtained using conventional stencils.
Another object of the present invention is to provide a method for producing a gravure roll having desired size and depth wells adapted for receiving liquid which can then be transferred to a receiving surface to produce a predetermined shape and shade of printed patterns on the receiving surface.
Another object of the present invention is to provide a method for producing a gravure roll having desired size and depth wells adapted for receiving liquid which can then be transferred to a receiving surface to produce a predetermined printed pattern without fuzzy edges defining said printed pattern.
The above and further objects and advantages of this invention will become apparent upon consideration of the following description thereof.